Of course, if the San Francisco 49ers go one-and-done in the playoffs or even miss the postseason entirely, Willis is probably going to play well—that’s what he does. Game in and game out, players like Willis, Joe Staley and Justin Smith provide a consistent level of excellence that the 49ers can rely on to be the foundation of their success.

This article is not about them.

This article is about San Francisco’s X-factors: players whose performance will be the difference between another Super Bowl appearance and a date on the couch come February. We’re looking for players whose fortunes have coincided with the team in general—players with good performances when San Francisco wins and bad performances when it loses.

The higher the highs and the lower the lows, the more likely they are to appear on this list. For the 49ers to make a deep run this year, they’ll need more of the former and a lot less of the latter.

Here are nine of San Francisco’s top X-factors entering the 2013 NFL playoffs.

Colin Kaepernick

When you look at the most important players for San Francisco, you have to start behind center.

Colin Kaepernick has been a bit of a letdown in 2013, with all things considered. After a blistering start to his career after he took over from Alex Smith, he was expected to continue right where he left off, if not get better. Instead, he’s had the equivalent of a sophomore slump, despite this being his third season.

In almost every statistical methodology, Kaepernick is below where he was last year. Completion percentage, interception percentage, yards per attempt, quarterback rating, ESPN’s QBR, Football Outsider’s defense-adjusted value over average (DVOA)—they're all lower in 2013 than 2012.

However, there’s a definite split between the wins and losses. When he is given time and gets into a groove, San Francisco’s offense starts to click.

It’s not a matter of yardage—although he is averaging 221.3 yards per game in wins and only 123.8 yards in losses. It’s a matter of control and minimizing negative plays. He has more interceptions and nearly as many sacks in less than half the number of games when San Francisco loses.

His running numbers are actually better when the team loses, but that’s because he’s being chased around the pocket. Rather than looking for people down field, he’s pulling the ball down and running.

In wins, Kaepernick has played like a top-10 quarterback, not miles off from someone like Andrew Luck. In those games, he finds chemistry with Anquan Boldin and Vernon Davis, stakes San Francisco out to a big lead and manages the game smartly the rest of the way.

In the losses this season, however, he has been as bad as any quarterback in football. He completes less than half his passes and spends more time on his back than in the pocket. That level of performance won’t cut it come playoff time. As Kaepernick goes, so will the 49ers.

He was listed as questionable against the Saints, but he played every offensive snap except for two; it's the only game the 49ers have lost this season where Davis didn’t miss time due to injuries.

Davis’ health has never been an issue for San Francisco before—the last game he had missed due to injury before this season was all the way back in 2007.

Had this happened last year, the 49ers would have turned to Delanie Walker, an all-around, do-everything veteran who is having a very solid season in Tennessee.

This year, however, they’ve had to go to rookie tight end Vance McDonald, who is struggling through his debut year. He is only catching 50 percent of the balls thrown his way and has yet to flash the athleticism that tempted the 49ers into taking him in the second round of this year’s draft.

When you add the lack of depth behind Davis to the lack of talent at the receiver position for most of this season, you can understand why he has been so important to the 49ers’ fortunes. If he ends up going down with an injury in the playoffs, San Francisco doesn’t have another way it can turn—it would cripple the passing offense.

Anquan Boldin

In his first season in the red and gold, Anquan Boldin has formed an immediate rapport with Colin Kaepernick. It’s a good thing, too. With both Michael Crabtree and Mario Manningham missing extensive time this year, the San Francisco quarterback needed a talented receiver like Boldin to boost the pass offense.

When he has had space to work, Boldin has been dynamic. Prorate those nine victories into a full season, and he would be on pace for 92 receptions and 1,355 yards, which would be his best season since 2005.

That Boldin would be a lock for the Pro Bowl; maybe he’d be a rung down from the Calvin Johnsons of the world, but it would be a fantastic season for a 33-year-old player. He is Kaepernick’s first option on almost every play and can always be counted on for a tough first down in traffic.

Unfortunately, the rest of the league caught on quickly to the fact that Boldin was, for vast stretches of the year, the entire pass offense, as opposed to just a key cog in it. When both Crabtree and Davis missed time simultaneously, Kaepernick had no other targets. Boldin found himself draped in coverage—he’d face double or triple-teams and didn’t have a way to shake them off and get open.

Prorate those four losses into a full season, and you get a wide receiver on pace for 60 catches for 612 yards, which does not exactly set the world on fire.

With the full contingent of receivers finally healthy for San Francisco, Boldin has returned to key-weapon status. Because opposing defenses have to cover somebody else as well, he had the most one-on-one coverages he’s seen since Week 1. Over the past month, he’s averaging six receptions a game for 85 yards and a touchdown.

Can he keep that pace going against the tough defenses he’ll face in the NFC playoffs? San Francisco needs that answer to be yes.

Michael Crabtree

How badly did the 49ers miss Michael Crabtree? Here are the full stats for 49ers receivers and tight ends not named Davis, Boldin or Crabtree:

Receiving

Targets

Receptions

Completion%

Yards

Touchdowns

Kyle Williams

27

12

44.4%

113

0

Vance McDonald

16

8

50.0%

119

0

Mario Manningham

19

8

42.1%

83

0

Jonathan Baldwin

8

3

37.5%

28

0

Quinton Patton

3

1

33.3%

0

0

Marlon Moore

3

1

33.3%

6

0

Garrett Celek

2

1

50.0%

30

0

Derek Carrier

0

0

0.0%

0

0

Kassim Osgood

0

0

0.0%

0

0

Thirty-four receptions, all year long. That’s it. There are 91 individual players in the NFL with more receptions than that; Cincinnati alone has five players above that mark. That’s not only failing to get it done, that’s not even worth paying attention to.

It’s no wonder Boldin and Davis have found themselves unable to get open.

Crabtree is not at 100 percent yet and likely won’t be until next season, but there’s nothing wrong with his hands, and having someone else out there who’s a threat to come down with a catch is a huge boon to the offense.

Already we’ve seen flashes of the old Crabtree over the past two games—plays where he suddenly bursts down field or leaps up to make an astounding catch. He doesn’t quite have the same explosive ability out of his cuts, but he is still leaps and bounds more dangerous than anyone else that San Francisco has trotted out this season.

His major role this postseason won’t be as a top receiver but as someone to keep the defense honest. If teams rotate too much coverage toward Boldin and Davis, Crabtree is healthy enough to make defenses pay.

For potential rematches against Carolina and New Orleans, both of whom were able to clamp down on the 49ers passing game in a Crabtree-less world, having him as an option could make a huge difference, even if the numbers go to other players.

Tarell Brown

Ezra Shaw/Getty ImagesHow will Tarell Brown perform, if he can return to the field?

Believe it or not, some players on the 49ers are not involved in the passing game. It might even be possible that some of them make an impact in the playoffs, amazingly enough.

Tarell Brown was having a solid season as San Francisco’s starting right cornerback. He was holding his assignments to a 58.6 percent completion rate and still leads 49ers cornerbacks with eight passes defensed, according to Pro Football Focus (subscription required). While he is yet to come down with an interception this season, he has only allowed two touchdowns and has played solid if unspectacular coverage.

In his absence, Tramaine Brock has stepped up and played brilliantly, but moving Brock to the starting lineup means Eric Wright’s moved into the nickel corner role. Wright came down with the game-clinching interception against Seattle, but not before he missed some time with an injury as well. For depth’s sake alone, if not for talent, the 49ers could use Brown back in the lineup sooner rather than later.

The injury doesn’t appear to be year ending, but San Francisco is taking its time with his recovery in an attempt to avoid any further complications or aggravations. With the potent passing attacks of Detroit and New Orleans lurking—San Francisco will have to beat both teams in back-to-back weeks on the road—the 49ers need all hands on deck in the secondary.

Will Brown return to action? If so, will he be able to play at a high level? It remains too early to tell.

Anthony Davis, Jonathan Goodwin, Alex Boone and Mike Iupati

Kirby Lee-USA TODAY SportsKaepernick can't pass, and Gore can't run if the offensive line doesn't do its job.

Offensive-line play is key to the offense's success, and in 2012, San Francisco had some of the best line play you will ever see. Last season, Football Outsiders ranked the San Francisco line as the best in the business when it came to running the football, with 4.50 adjusted line yards. The O-line also ranked tops in getting to the second level—getting those blocks on linebackers to create further lanes downfield.

This season, those numbers have slipped somewhat, with the 49ers falling down toward the back of the pack. While that overstates the degree of the drop-off from last season, the line hasn’t looked as good in 2013, any way you slice it.

Left tackle Joe Staley has been a rock and should be headed back to the Pro Bowl. The other four linemen have had their struggles. None of the four have been plain bad all season long or anything of that nature—they’ve just been inconsistent.

Alex Boone has been all over the place on Pro Football Focus’ charting, with five negative games matched up against four positive ones. He ends up computing out as being somewhere around average. He’s only given up two sacks all year long, but his grades show him to be the picture of inconsistency. He’ll back up a poor game in Tennessee, where he was unable to generate any sort of push, with a game against Jacksonville, where he easily stepped into the second level.

Anthony Davis has pulled it together after a wild, up-and-down start to his season. His last three games have all charted out into the positives, and he hasn’t allowed a sack since the New Orleans game in Week 11. Early on this season, he was having problems in the run game, with runs to right tackle only averaging 3.4 yards per clip, but that has balanced out some, too. Is it too small of a sample size in three weeks, or has he cleaned up his play?

Jonathan Goodwin has also looked much better over the past three weeks. He’s only graded out significantly positively four times this season, but they’ve all been since Week 8—in other words, he’s trending upward as the season progresses.

Mike Iupati has the lowest overall grade of any of the linemen, but that’s mostly the product of a nightmare of a game in Seattle, where he gave up six quarterback hurries. It took opponents a month-and-a-half after that game to ring up six more on Iupati. That’s not to say it was a fluke; after a Pro Bowl season in 2012, he has been allowing more hurries from his side and has missed the last three weeks with a sprained MCL. His ability to come back into the line without missing a beat will be key.

Again, it’s not that it’s been a bad line per se, just bad compared to the level we know these guys played at in 2012. Over the last three weeks, they’ve begun to look more like that line, and the 49ers' fortune has improved as a result.

If it’s just a fluke or a small sample-size mirage, then San Francisco could struggle against tough defensive fronts like Carolina and Detroit come playoff time. Hopefully for San Francisco, it’s not just noise—it’s a sign that the team is jelling at precisely the right time.